1. Field of the Invention
This invention relates to improvements in heat transfer tubes and tube assemblies for a water-spray panel evaporator for the gasification of liquefied natural gases and a method for maufacturing such improved tubes and tube assemblies. More particularly, the invention relates to heat transfer tubes with improved heat transfer performances, assemblies of such tubes and a method for manufacturing such tubes and tube assemblies.
2. Description of the Prior Art
The liquefied gas evaporator of the water-spray type is an evaporator having a liquefied gas guided through the interior of heat-transfer tubing in an upward direction while a heating medium such as seawater or industrial water is sprinkled against the exterior of the heat transfer tubing, whereby the liquefied gas is heated across the heat transfer tubing and, hence, gasified. This type of evaporator has several features unshared by other gasification systems.
The pattern of heat exchange between materials having a very large temperature difference, for example between a liquefied natural gas and water, where the former has a temperature of, for example, -150.degree. to -200.degree. C. and the latter has a temperature of, for example, 5.degree. to 20.degree. C. is significantly different from the normal pattern of heat exchange and, even in the case of boiling heat transfer which is generally acknowledged to provide an excellent heat transfer performance, the film boiling that takes place under such circumstances causes a reduction in heat transfer performance, thus thwarting attempts to obtain a satisfactory heat transfer performance.
By way of illustration, FIG. 1 is a schematic diagrammatic representation of the normal gas-liquid binary-flow heat transfer pattern providing a satisfactory heat transfer performance, obtained when a liquefied gas AL to be gasified is fed into the interior of a heat transfer tube 1 and water B as a heating medium is passed externally of the tube 1 to accomplish an exchange of heat therebetween. In the above arrangement, the liquefied gas AL in tubing 1 normally presents a vapor-liquid binary flow, with the gas AG produced by heat exchange progressively departing from the tube wall and ascending in the central zone of the tubing and the liquefied gas AL adjacent the gas-liquid boundary being drawn up to the tube wall in a sectional configuration approximating the letter V as illustrated in FIG. 1. Since, in this situation, there is adequate contact between the heat transfer tubing liquefied gas AL, a high heat transfer performance and, hence, a satisfactory evaporation effect are realized.
However, when the material to be evaporated is a material having a very low boiling point such as a liquefied natural gas, the binary flow situation depicted in FIG. 1 does not prevail because of the extremely large temperature difference between the evaporation load and the heating medium. Rather, a so-called film boiling takes place to cause a sharp reduction in heat transfer performance.
This film-boiling heat transfer situation is schematically shown in FIG. 2, where the liquefied gas AL undergoes film-boiling at the tube wall and is quickly gasified with the liquefied gas AL being centrally distributed in the form of a cone within the tubing 1. Since the area of contact between liquefied gas AL and tubing 1 is too small to provide adequate heat transfer, the heat transfer performance is considerably affected. To overcome the above disadvantages, it has been proposed to insert a twist-tape into the heat transfer tube 1 to create a spiral upward flow of liquefied gas A and to thereby disrupt the liquid cone and cause the liquid mist particles to impinge on the tube wall. In this connection, it has also been proposed to increase the heat transfer area by providing the internal surface of the tube with dimples or fins. Nonetheless, as far as a water-spray type panel evaporator for liquefied natural gas is concerned, there is much to be desired with respect to efficiency and other performance parameters.
This invention has been accomplished under the circumstances briefly described above.